Internal combustion engines having a relatively small number of cylinders provide automobile makers with an attractive solution to the need for improved fuel economy. In order to compensate for reduction of cubic capacity, vehicle manufacturers developed technologies to improve engine power, such as direct fuel injection, turbocharging, and variable timing for inlet and exhaust camshafts. In this way six- and eight-cylinder engines can be scaled down without losing available horsepower.
An undesirable consequence of engines with a small number of cylinders is high crankshaft torsional vibration and high engine block vibration caused by forces, such as first and second engine order forces, that are not cancelled. Such torsional vibrations are ultimately transmitted through the engine mounts and to the vehicle structure.
Engineers managed these vibrations to one extent or another through a variety of approaches, many of which increase the cost of construction and reduce fuel economy. One accepted solution to overcome excessive vibration is the provision of one or more pendulums on the crankshaft to lower the torsional vibration of the crankshaft and the consequent vehicle noise and harshness. Such crankshaft-mounted pendulums function as vibration absorbers as they are tuned to address and thus reduce vibrations generated by oscillating torque, thereby smoothing torque output of the crankshafts. This approach is taken as well by designers of some airplane piston engines where the pendulums smooth output torque and reduce stress within the crankshaft itself.
An example of a pendulum vibration absorber associated with an engine crankshaft is set forth in U.S. Pat. No. 4,739,679, assigned to the assignee of the instant application. According to the arrangement set forth in this patent, a pendulum includes an inner curved cam follower surface that is alternately engaged and disengaged from a pin type cam fixed on the pendulum carrier.
The crankshaft pendulum is interconnected with the pendulum carrier by pairs of rollers that are movable on mating curved tracks. While there are a number of variations of the movable relationship between the pendulum and the crankshaft, it is common to incorporate rolling pins as the points of contact between these two components.
Each rolling pin requires a pendulum rolling pin track in which the rollers can roll. Known rolling pin tracks have great distances between the walls of the track and the rolling pin. When the engine is running and the crankshaft is rotating, centrifugal force keeps the pendulum in its outward position. However, when the engine is turned off and rotational movement of the crankshaft stops, centrifugal motion stops as well and the pendulum, no longer held in its fully outward position, may experience a drop caused by gravity if the stopped position of the pendulum is “up” or is generally above the midline of the crankshaft. If the pendulum is stopped in this position, then it will drop a distance of over 3.0 mm before hitting metal-on-metal, thus increasing undesirable NVH in the engine and, consequently, in the vehicle.
To compensate for this drop, rubber bumpers are strategically located on the pendulum carrier to dampen the metal-on-metal contact. The use of such corrective measures not only adds to manufacturing and material cost, but also creates a risk of clogged oil lines by particles of rubber due to the degradation of the rubber over time.
Thus a new approach to the pendulum crankshafts is needed to address the problems associated with known arrangements.